1. Field of the Invention
The present invention relates to zoom lenses and, more particularly, to zoom lenses of the retrofocus type.
2. Description of Related Art
For a camera using an image-pickup element, such as a video camera or digital still camera, a certain long back focal distance is prerequisite to the photographic lens, for the single-lens reflex camera, to a wide-angle lens, and for an liquid crystal projector, to the projection lens. It has, therefore, been the common practice in the art that a lens unit of negative refractive power takes the lead as viewed from the long-distance conjugate point side (in the camera, from the object side, or in the projector, from the screen side). That is, the retrofocus type has been used in such lenses.
Zoom lenses of the retrofocus type for such cameras are known in Japanese Laid-Open Patent Applications No. Hei 1-191820, No. Hei 3-203709 and No. Hei 3-240011. Each of these comprises, in order from the object side, a first lens unit of negative refractive power, a second lens unit of positive refractive power and a third lens unit of positive refractive power, the second lens unit axially moving to vary the focal length, while simultaneously moving the first lens unit to compensate for the shift of an image plane.
A projection lens of the retrofocus type for the liquid crystal projector in Japanese Laid-Open Patent Application No. Sho 62-291613 comprises a first lens unit of negative refractive power followed by a number of lens units whose overall refractive power is positive. Japanese Laid-Open Patent Application No. Hei 3-145613 discloses another projection lens which comprises, in order from the screen side, a first lens unit of negative refractive power, a second lens unit of positive refractive power and a third lens unit of positive refractive power.
To simplify the aberrational problems, it has been well known to use aspheric surfaces in the lens system. The aspheric surface is very effective for correcting spherical aberration, curvature of field and coma, but has a little effect on chromathc aberrations. To correct chromatic aberrations, a diffractive optical element can be used as disclosed in Japanese Laid-Open Patent Application No. Hei 4-213421 (U.S. Pat. No. 5,044,706), Japanese Laid-Open Patent Application No. Hei 6-324262 and U.S. Pat. No. 5,268,790 and No. 5,493,441.
However, the lens systems disclosed in Japanese Laid-Open Patent Applications No. Hei 4-213421 and No.
Hei 6-324262 are monofocal. Therefore, no consideration is made on the chromatic aberrations of the zoom lens, in which their variation is very peculiar. Meanwhile, U.S. Pat. No. 5,268,790 and U.S. Pat. No. 5,493,441, although disclosing examples of application of the diffractive optical element to zoom lenses, both do not relate to the retrofocus type. So far, there are not known any examples of application of diffractive optical elements to the retrofocus type zoom lens.